Thin films are often produced using plasma sputtering techniques. Targets are used during the sputtering process as the source of material for the film being deposited onto a substrate. Targets made from volatile and thermally unstable metal oxides have been used to sputter thin films which exhibit properties useful in a variety of applications. For example, thin films of electro-optic materials, such as indium oxide and tin oxide, are known to exhibit high transmittance and low resistivity. These materials are commonly used as electro-conductive films in electroluminescence (EL) displays, liquid crystal displays, solar cells, defrost/defog heaters for airplanes and the like.
Previous methods used to make sputtering targets from such volatile and thermally unstable metal oxides have included consolidating, such as by hot-pressing, powders of the metal oxide target starting material, like In.sub.2 O.sub.3 and SnO.sub.2, into a target blank in an inert gas environment. Graphite or ceramic die assemblies with a die cavity have typically been used to press the powdered material. With a number of these prior processes, in particular prior processes for making indium oxide and tin oxide (ITO) targets, the metal oxide powder was loaded into the die cavity such that the powder was in direct contact with the walls of the die cavity. After consolidation, the resulting target blank was typically formed, such as by machining, grinding, polishing, etc., into a finished target. Detailed descriptions of a number of prior processes for making ITO sputtering targets may be found in U.S. Pat. Nos. 5,160,675 and 5,094,787 and in Japanese Patent Nos. 04341504, 04293769, 04154654, 04074860, 03199373 and 02043356.
The production of metal oxide targets, notably ITO targets, using graphite die assemblies have been known to exhibit a number of chronic problems associated with interaction between the powdered target starting material and the graphite material of the die assembly. The problems have included the full reduction of the metal oxide to metal, at least in the form of a layer on the outer surface outside of the resulting target blank. Removal of this metal layer is generally necessary before the target is suitable for use. However, removal of the reduced metal, for example, a layer of indium or indium-tin alloy on an ITO target, often results in cracks initiating in the target or the target actually fracturing and having to be scrapped. The formation of cracks in the target reduces its resistance to fracturing during use. Lower pressing temperatures and/or shorter times at temperature have been used in order to avoid the formation of a reduced metal layer while still using a graphite die assembly to press the powder. Pressing at lower temperatures and/or shorter times typically results in poor consolidation of the powdered target starting material. Such poor consolidation typically results in targets with low densities, low strength and/or a low resistance to fracturing (i.e., low toughness).
The density, strength and toughness of a target is often very important to the target's performance. Reportedly, high density ITO targets are required in order to sputter high quality ITO thin films, in particular, thin films free from particles. In addition, good strength and toughness are typically necessary to successfully form the ITO target blank into the desired target shape. ITO targets with good toughness are also less likely to fracture during the sputtering process. Such fracturing can result in relatively large particles being deposited onto the substrate, often generating defects in the ITO film. Such defects in the ITO film can adversely affect the electro-optic properties of the film.
ITO and other such metal oxide sputtering targets produced with die assemblies made of a ceramic, such as Al.sub.2 O.sub.3 or ZrO.sub.2, are less likely to exhibit the problems noted above when graphite die assemblies are used. However, such ceramic die assemblies are generally more expensive to manufacture, less resistant to thermal shock and not as thermally conductive as comparable graphite die assemblies. Ceramic dies are thus more likely to crack during use, require longer heating and soaking times during the hot-pressing operation, and generally increase the costs of the target manufacturing process.
Therefore, there is a need for a more cost effective method of manufacturing denser, stronger and tougher sputtering targets made from volatile and thermally unstable metal oxides.